1. Field of the Invention
The present invention is related generally to a method of treating a nickel titanium alloy, known as Nitinol, for use in manufacturing instruments having improved resistance to cyclic fatigue failure. As a particular application, the invention is related to preparation of Nitinol wire blanks for use in manufacturing endodontic files having improved resistance to cyclic fatigue failures.
2. Background of the Invention
Many medical applications take advantage of the properties of Nitinol, a nickel and titanium alloy. Nitinol (an acronym for Nickel Titanium Naval Ordinance Laboratory) exhibits several useful properties such as shape memory, by which a Nitinol component returns to a previously memorized shape after being forced into a second shape. Nitinol also exhibits superelasticity, meaning that a Nitinol component may be deformed elastically to a very large extent by strain without reducing its ability to return to the its original shape after the strain has been removed. One drawback of Nitinol, however, is that in certain configurations it is not very resistant to fatigue, i.e. repeated cyclic strains.
In the specification, claims and abstract that follows the term “martensite” means an alloy in a substantially martensitic phase or condition and the term “austenite” means an alloy in a substantially austenitic phase or condition. Therefore the term “martensite state” means a “substantially martensite state” and the term “austenite state” means a “substantially austenite state”.
The present invention is directed to a method of preparing Nitinol so that it can be used to manufacture instruments that retain the martensitic state at the operating temperature with corresponding greater resistance to cyclic fatigue failure.
The present invention is further directed to a method of forming a dental device comprising the steps of forming the device of Nitinol having an impressed memorized shape, wherein the memorized shape is a shape the element assumes when in an operational configuration. The element is treated so that it is substantially martensite phase stabilized under expected operating conditions.
Nitinol is an alloy which was developed to achieve improved elasticity and other enhanced mechanical properties. Nitinol also possesses shape memory properties that are well suited for medical and dental applications. Elements constructed of Nitinol may be formed in a first “memorized” shape to which they will return after deformation. That is, when such a Nitinol element has been deformed, raising a temperature of the element above a critical temperature causes the element to revert to its memorized shape.
As would be understood by those of skill in the art, Nitinol alloys can exist in one of two different temperature-dependent crystal structures. At lower temperatures, Nitinol is martensitic, meaning that its structure is composed of self-accommodating twins, in a zigzag-like arrangement. Martensite is soft and malleable, and can be easily deformed by de-twinning the structure via application of strain. At higher temperatures, above a critical temperature of the alloy, Nitinol is austenitic. Austenite is a strong and hard phase of the alloy, exhibiting properties similar to those of titanium, and is characterized by a much more regular crystalline lattice structure. Nitinol alloys can also undergo a phase change as a result of the application of a strain. For example, an element in the austenitic phase can be bent so that at high strain locations the alloy becomes martensitic. If the alloy is designed to have an unstable martensite phase at the operating temperature, removal of the strain results in a reverse transformation that straightens the bending.
3. Description of the Prior Art
For background information relating to the subject matter of this invention, reference may be had to the following issued United States patents and publications:
PAT. NO.INVENTOR(S)ISSUE DATETITLE5,464,362Heath et al.Nov. 07, 1995Endodontic Instrument5,762,541Heath et al.Jun. 09, 1998Endodontic Instrument5,984,679Farzin-Nia et al.Nov. 16, 1999Method of Manufacturing SuperelasticEndodontic Files and Files Made Therefrom6,149,501Farzin-Nia et al.Nov. 21, 2000Superelastic Endodontic Instrument, Methodof Manufacture, and Apparatus Therefor6,315,558Farzin-Nia et al.Nov. 13, 2001Method of Manufacturing SuperelasticEndodontic Files and Files Made Therefrom6,428,317AbelAug. 06, 2002Barbed Endodontic Instrument6,431,863Sachdeve et al.Aug. 13, 2002Endodontic Instruments Having ImprovedPhysical Properties6,626,937CoxSep. 30, 2003Austenitic Nitinol Medical Devices2003/0,199,236Aloise et al.Oct. 23, 2003Method of Manufacturing An EndodonticInstrument2004/0,171,333Aloise et al.Sep. 02, 2004Method of Manufacturing An EndodonticInstrument2004/0,193,104JervisSep. 30, 2004Bendable, Reusable Medical Instruments WithImproved Fatigue Life2004/0,216,814Dooley et al.Nov. 04, 2004Shape Memory Alloy Articles With ImprovedFatigue Performance and Methods Therefore2005/0,059,994Walak et al.Mar. 17, 2005Fatigue Resistant Medical Devices2005/0,090,844Patel et al.Apr. 28, 2005Long Fatigue Life Nitinol